Hi all

I received my FL-33 9.01MHz IF filter yesterday and quickly installed
it into my R71, following all the instructions in Don Moman's
excellent R71 Performance Manual. The thing is, I can't hear any
difference with it installed. I was wondering if anyone knows a way to
check if it's actually working or not.

I replaced the stock 455KHz 3rd IF 6KHz ceramic filter with an LFH4S
from Kiwa a few months ago. This has a -6dB bandwidth of 6KHz, a shape
factor of about 1:1.7 and an ultimate rejection of 70-75 dB. The FL-33
also has a -6dB bandwidth of 6KHz and a shape factor of about 1:2. It
has an ultimate rejection of about 90dB. Because the Kiwa is slightly
tighter, I didn't expect to hear much difference on any but the most
powerful signals, but while switching between the on and off positions
on the filter switch on the main board, I could detect no difference
at all. Even removing and re-inserting the FL-33 produced no
detectable difference to my ears, or to the reading on the S meter.

I had imagined that the filter would make a difference in ultimate
rejection. To test this, I tuned in the strongest AM signal on Medium
Wave, tuned about 5 or 6KHz away until I could only hear a bit of
splatter, and then flicked the filter switch between on and off.
Again, no difference between the Kiwa alone, and the Kiwa preceded by
the FL-33.

I used short wires between the snipped ends of resistors R58 and R52,
as recommended in the performance manual, and this had the result I
expected, namely causing a 2KHz offset in the narrow AM mode, but I'm
wondering if I should be able to hear a difference with the FL-33 in
AM wide.

Any help or suggestions would be most appreciated.

Thanks

Andy

Hi all

I think I've cracked it and realised how wrong I was initially.

I've just been playing around with the receiver doing some tests.
Basically, I attached a jump wire across the input and output pins
of my 455 Khz IF Kiwa filter to bypass it, and determined that
selectivity was still ok, but not quite as tight as with the Kiwa
filter in full operation. I then added a jump wire across the input
and output pins of the Fl-33. Selectivity went from good to terribly
wide. I then realised that the filter switch only affects the CW
narrow mode.

I feel a bit stupid but I'm glad I solved the problem. After
bypassing the filters in various combinations, I can say that the
combination of FL-33 + Kiwa LFH4S is definitely better than either
filter on its own. The Kiwa is an excellent filter on its own,
though and if I had to keep only one of the two, I'd choose the Kiwa.

Anyway, thanks for your help.

Cheers

Andy